Information on human settlements is crucial for a wide range of applications including emergency response, disaster risk reduction, population estimation/analysis, and urban/regional planning. Urbanization pressure generates environmental impacts, indicates population growth, and relates to risk and disaster vulnerability. For instance, the global population passed the mark of 7.0 billion in 2011 with more than half of the population living in urban areas. Between 2011 and 2050, the urban population is expected to increase by about 2.7 billion, passing from 3.6 billion in 2011 to 6.3 billion in 2050. The population growth in urban areas is projected to be concentrated in the cities and towns of the less developed countries and continents. Asia, in particular, is projected to see its urban population increase by 1.4 billion, Africa by 0.9 billion, and Latin America and the Caribbean by 0.2 billion.
Population growth is therefore becoming largely an urban phenomenon concentrated in the developing world resulting in major challenges to manage the urban development in a sustainable manner. One issue in this respect is the availability of up-to-date information on the extent and quality of the urban settlement (e.g., the urban “built-up” or “build-up,” such as man-made 3-dimensional structures) which is largely unavailable in developing countries. For instance, cities are often growing at a pace that cannot be fully controlled by the local or regional mapping agencies. As demographic pressure increases exponentially at a global level, the ability to monitor, quantify and characterize urbanization processes around the world is becoming paramount. The information about the quality of urban development can provide precious input for understanding the vulnerability of the population living on our planet.
While overhead imagery could provide information about the world-wide built-up environment, there are few global data sets available that could be used to map the human settlements. Examples include the night-time lights of the world based on the Defense Meteorological Satellite Program—Operational Linescan System (DMSP-OLS) sensor, Moderate Resolution Imaging Spectroradiometer (MODIS) based land use/land cover classifications, and global population data sets like LandScan™ or the gridded population of the world (GPW). In the case of LandScan and GPW, for instance, available global population data sets include population density estimation cell sets at coarse resolutions of 0.00833 (approximately 1000 m) or the like (e.g., where each population density estimation cell is in the form of a square that represents 1000 m on a side and indicates a particular population density or population density range over a geographic region, city etc.). While such population density data sets may be somewhat useful in conveying general population density estimates over large areas, they are limited in their ability to provide more fine grained population density estimates. In the case of an urban area, for instance, population densities may vary widely just between adjacent street blocks.